The present invention relates to a heater for the pyrolysis of hydrocarbons and particularly to a heater for the steam cracking of paraffins to produce olefins.
The steam cracking or pyrolysis of hydrocarbons for the production of olefins is almost exclusively carried out in tubular coils located in fired heaters. The pyrolysis process is considered to be the heart of an olefin plant and has a significant influence on the economics of the overall plant.
The hydrocarbon feedstock may be any one of the wide variety of typical cracking feedstocks such as methane, ethane, propane, butane, mixtures of these gases, naphthas, gas oils, etc. The product stream contains a variety of components the concentration of which are dependent in part upon the feed selected. In the conventional pyrolysis process, vaporized feedstock is fed together with dilution steam to a tubular reactor located within the fired heater. The quantity of dilution steam required is dependent upon the feedstock selected; lighter feedstocks such as ethane require lower steam (0.2 lb./lb. feed), while heavier feedstocks such as naphtha and gas oils require steam/feed ratios of 0.5 to 1.0. The dilution steam has the dual function of lowering the partial pressure of the hydrocarbon and reducing the carburization rate of the pyrolysis coils.
In a typical pyrolysis process, the steam/feed mixture is preheated to a temperature just below the onset of the cracking reaction, typically 650xc2x0 C. This preheat occurs in the convection section of the heater. The mix then passes to the radiant section where the pyrolysis reactions occur. Generally the residence time in the pyrolysis coil is in the range of 0.2 to 0.4 seconds and outlet temperatures for the reaction are on the order of 700xc2x0 to 900xc2x0 C. The reactions that result in the transformation of saturated hydrocarbons to olefins are highly endothermic thus requiring high levels of heat input. This heat input must occur at the elevated reaction temperatures. It is generally recognized in the industry that for most feedstocks, and especially for heavier feedstocks such as naphtha, shorter residence times will lead to higher selectivity to ethylene and propylene since secondary degradation reactions will be reduced. Further it is recognized that the lower the partial pressure of the hydrocarbon within the reaction environment, the higher the selectivity.
The flue gas temperatures in the radiant section of the fired heater are typically above 1,100xc2x0 C. In a conventional design, approximately 32 to 40% of the heat fired as fuel into the heater is transferred into the coils in the radiant section. The balance of the heat is recovered in the convection section either as feed preheat or as steam generation. Given the limitation of small tube volume to achieve short residence times and the high temperatures of the process, heat transfer into the reaction tube is difficult. High heat fluxes are used and the operating tube metal temperatures are close to the mechanical limits for even exotic metallurgies. In most cases, tube metal temperatures limit the extent to which residence time can be reduced as a result of a combination of higher process temperatures required at the coil outlet and the reduced tube length (hence tube surface area) which results in higher flux and thus higher tube metal temperatures. The exotic metal reaction tubes located in the radiant section of the cracking heater represent a substantial portion of the cost of the heater so it is important that they be utilized fully. Utilization is defined as operating at as high and as uniform a heat flux and metal temperature as possible consistent with the design objectives of the heater. This will minimize the number and length of the tubes and the resulting total metal required for a given pyrolysis capacity.
In the majority of cracking furnaces, the heat is supplied by hearth burners that are installed in the floor of the firebox and fire vertically up along the walls. Because of the characteristic flame shape from these burners, an uneven heat flux profile is created. The typical profile shows a peak flux near the center elevation of the firebox, with the top and bottom portions of the firebox remaining relatively cold. In select heaters, radiant wall burners are installed in the top part of the sidewalls to equalize the heat flux profile in the top. Typical surface heat flux profiles and metal temperature profiles for a hearth burner and for a combination of hearth and wall burners at the same heat liberation rate show low heat flux and metal temperature in the lower portion of the firebox, which means that the coil in this portion is underutilized. Improving the hearth burner flux profile is difficult because of the additional NOx requirements, and because of the steadily increasing demand of higher burner heat releases. Another way to equalize the flux profile is using wall burners only, but since the maximum heat release of a wall burner is about 10 times less than that of a hearth burner, the number of burners would become excessive.
The present invention relates to pyrolysis heaters, particularly for the cracking of hydrocarbons for the production of olefins, with a burner arrangement in the firebox to improve the heat flux and metal temperature profile. The objective is to provide a burner arrangement which includes burners to heat the floor of the firebox so that it acts as a radiant surface to increase the heat flux to the reaction tubes in the lower part of the firebox and produce a more uniform vertical heat flux profile over the firebox elevation. These floor burners are called base burners and work along with vertically firing hearth burners and optionally with wall burners in the upper portion of the firebox. It is a further objective to increase the total heat transferred to the radiant cracking coil with no increase in the metal temperature of the coil.